I don't think that they're unreasonable assumptions though. After all, even hypotheses are all based off assumptions (which are then tested with research or experimentation).
My assumptions are based off my own experience. As someone who works with teenagers I witness the effects this on a regular basis. And at schools we are seeing an ever increasing rate of things like:
* Students refusing to stop using their phones in class. Teachers are sometimes even abused when telling students to put their phones away.
* We are unable to confiscate phones from students because
parents complain
* I have taken students on trips, including overseas, where some students miss out on the marvels that I'm trying to show them because they're too busy staring at their screens
* Whenever I walk around the school during recess and lunch and I see groups of kids just using their devices and ignoring each other, I refer to these groups as "hunchbacks anonymous."
* Some medical experts have noted an increased rate of neck and upper spine issues with young people due to prolonged periods of being hunched over. Remember how our parents told us to sit with correct posture?
* Mental health: we are seeing an increased rate of kids who are have deteriorating interpersonal social skills.
* Mental health: we are seeing an increasing rate of kids with poor resilience skills. Counsellors, child psychologists and other mental health professionals are becoming increasingly inundated with cases.
* Mental health: we are seeing an increasing rate of social withdrawal from kids. At school we see this manifest in the form of school refusal, which then leads to police home liaison officers paying visits to encourage these kids to come to school, but because they're so mentally distraught we have to work out a programme for them to
slowly reintegrate into society and learn how to come back to school. This is often done by having them attend school on a part time basis and then reassessing them to see if they are mentally fit to attend full time. Needless to say that this has irrevocable effects on their education as they can miss many months of lessons and assessment tasks etc.
Remember that a child's brain is still incredibly malleable and that these changes to behaviour can literally rewrite their brain's architecture. As a result we are seeing increasing rates of kids with mental health diagnoses like ADD, ODD etc. And it's not even got to do with the devices themselves but basically just not spoiling the child rotten.
There have been a few times when I have been able to speak with these parents, and in each of these instances I have found no real excuse for them to allow the child to use the device at the table. It is, really, due to the parents being lazy. One classic case was at a friend's birthday where I saw another family spoon feeding their almost 2 year old.

When I spoke to them about this, the mother said, "Watch what happens when I take the phone away," and the kid just had a massive tantrum!

And she admitted that it was because this is the way they had conditioned the child. He has come to not only expect access to a device when eating, but he
demands it as entitlement.

I asked her, "Who's in control here, you are your kid?" and she fully admitted that it was her child who has authority over her, not vice versa. And IMO herein lies the problem. Parents should have authority over their children, not the other way around. That's not to say that children shouldn't be heard or respected, of course they should, but ultimately the authority lies with the parents.
Letting a child access a device whenever or wherever s/he wants... imagine that happening with junk food? Kids like sweets. Would you let them have sweets for breakfast, lunch and dinner? Would you let them eat Maccas
anytime they demanded it? And yeah, kids will kick up a stink if they don't get what they want sometimes. It happens with my kid too. The difference is that I don't allow it. Even if I think it might be appropriate, if my daughter
demands something rather than politely asking then the answer is no.
But hey, my experiences are just anecdotal, so let's look at what research data is telling us.
Smartphone use linked with increase in mental illness in adolescents
Is your child addicted to mobile devices?
Study concludes that we need more data for the effects of mobile device exposure in early childhood
P.S.: As the proverb goes,
"It takes a village to raise a child."